Certain present-day applications require analog-to-digital converters having a high sampling frequency and a high resolution. Among these applications are cable TV transmissions, optical communications and satellite communications in which one or more modulated signals are transmitted simultaneously on a wide frequency band (for example several GHz wide).
Moreover, the use of increasingly complex modulations requires a digitizing of the signals using a large number of bits.
Single-converter architectures have difficulty in meeting the required performance levels at an acceptable cost in terms of consumption and of space requirement.
Moreover, structures having several time-interleaved analog-to digital converters (TIADCs: Time-Interleaved Analog to Digital Converters) are being imposed in order to meet this requirement. In such a time-interleaved structure, the time-interleaved analog-to-digital converters respectively carry out time-shifted analog-to-digital conversions of an analog signal. In other words, if the structure comprises M converters, the latter successively sample, each in their turn, the signal at a frequency equal to Fs/M, where Fs is the overall sampling frequency of the structure.
However, the disadvantage of this type of structure is that mismatches, even slight, between the converters create parasitic lines or frequency bands that can be situated in the frequency zone of the useful signal. Moreover, in the case, for example, of television signals containing several channels, these parasitic bands or lines can interfere with some of these channels.
These mismatches can have various causes, such as for example timing skews and possibly static gains and/or offsets that are different between the converters of the structure.
Another cause of mismatch is the inequality between the values of the bandwidths of the different converters of the structure.
It is in fact known to those skilled in the art, for example from the article by Tsung-Heng Tsai and others entitiled “Bandwidth Mismatch and its Correction in Time-Interleaved Analog-to-Digital Converters,” IEEE Transactions on Circuits and Systems—II: Express Briefs, vol. 53, no. 10, October 2006, that an analog-to-digital converter can be represented in the first order by a first order low-pass filter and by an ideal sampler. The bandwidth of the converter is then defined by the cut-off frequency or the time constant of this first order low-pass filter. It is then said that there is bandwidth mismatch of the different converters of the structure when the different cut-off frequencies or different time constants of the different first order low-pass filters are different.
In practice, the cut-off frequency of these filters is high, typically of the order of one GHz or of about ten GHz, in order to prevent a cutting off of the useful signal. Such a difference in cut-off frequencies causes frequency bands or parasitic tones in the sampled signal. Moreover, as the input signal becomes higher and close to the cut-off frequency of the filters, the power of the parasitic frequency bands becomes greater with respect to the useful signal. This is particularly critical for analog input signals having high frequencies, typically of the order of a few tens of GHz, such as those found for example in optical communications.
The abovementioned article proposes a solution for correcting the bandwidth mismatch based on the hypothesis that the different time constants of the low-pass filters of the converters are already known, which is a limited solution.
The article by Shahzad Saleem and others, entitled “Adaptative Blind Background Calibration of Polynomial-Represented Frequency Response Mismatches in a Two-Channel Time-Interleaved ADC,” IEEE Transactions on Circuits and Systems—I: Regular Papers, vol. 58, no. 6, June 2011, deals with bandwidth mismatches of any order greater than or equal to 1.
In this second article, the solution described for dealing with these mismatches is valid only for an input signal having strict constraints in terms of frequency spectrum and bandwidth. The frequency spectrum must thus notably exhibit “holes” and the proposed solution aims to minimize the power of the parasitic frequency noise, appearing in these holes because of the mismatches, by way of iterative and retroactive corrections (that is to say carried out upstream of the converters). Here again, such a solution is limited and is moreover complex.